1. Field of the Invention
The present invention pertains to elevatable platforms or scaffolds by which workers may be positioned at various elevations relative to the ground or by which materials may be lifted above the ground. Specifically, it pertains to self-propelled or self-elevated platforms or scaffolds.
2. Description of the Prior Art
There are a number of types of self-propelled or self-elevated scaffolds, platforms or lifts. In one type of scaffold, called a scissor-life, pairs of structural members are arranged in scissor or parallelogram fashion so that as the horizontal distance between the ends of the structural members are shortened, the vertical distance is increased. In the past, such scaffolds have been propelled by hydraulic cylinders attached to a base or carriage. As the hydraulic cylinders are extended, the elevation of the work platform is increased. Nearly all currently manufactured elevating scaffolds utilize hydraulic cylinders. However, due to the large forces required in such arrangements and due to the weight of the hydraulic cylinders, such scaffolds or lifts are relatively heavy and expensive.
Prior to the development of hydraulic elevating scaffolds or lifts, some cable or chain driven lifts were developed. In such lifts, a cable or chain extends from a winch along one member or tong of a scissor arrangement and from the end thereof to the end of another scissor arrangement directly thereabove. Winding of the cable or chain on the winch causes the ends of the scissor assemblies to be drawn together and forcing the platform or scaffold mounted on the scissor assemblies to be elevated. However, one of the problems in such an arrangement is the initial movement of the scissor structure from a totally collapsed position toward extended positions. In the totally collapsed position, all portions of the cable or chain are essentially in a horizontal position so that there are no vertical force components to initially lift the scissor mechanism to an extended position.
This problem has been approached in several different fashions. For example, in U.S. Pat. No. 1,078,759, an auxiliary elevating device is provided which includes a pair of blocks mounted for vertical movement in guideways. The blocks are connected by a rod so as to press on the lower tongs of the scissor mechanism. Attached to the blocks is a rope or cable which extends around a pulley and, in some manner, around a shaft. The shaft is driven by a chain and sprocket which is selectively activated by a lever and gear arrangement. When this arrangement is in gear, the cable lifts the blocks and consequently the scissor assembly until it is started on its upward movement. Then the lever is shifted and the blocks placed in a passive position. This arrangement is rather bulky and complex.
In U.S. Pat. No. 1,817,418, initial movement of the scissor mechanism from the collapsed position is accomplished by providing arms and pulleys extending from tongs or linkages. In the completely collapsed position, these arms depend downwardly so that the pulleys engage the cable in such a manner that the cable stretches or extends in a "V" form. This provides the necessary vertical forces to initiate movement of the mechanism. Once the mechanism is partially elevated, the vertical assistance is discontinued and horizontal forces take over for further elevation. However, this type of arrangement requires that the base be elevated enough to accommodate the downwardly depending arms. Therefore, the entire elevatable scaffold or lift must be elevated or permanently placed on an elevated platform such as a truck or the like.
It is apparently this problem of initiating vertical movement that has caused elevatable scaffolds or platforms to evolve toward hydraulic lift mechanisms even though such hydraulic lift platforms are relatively more bulky, heavy and expensive. Thus, there appears to be no presently commercially accepted cable driven self-elevated scaffolds.